Dr. James Brewster (Bela Lugosi) and his colleague Dr. Randall (Henry Hall (actor)) are involved in a series of scientific experiments which have caused him to transform into an ape-man. In an attempt to obtain a cure Brewster believes that it will be necessary to inject himself with recently drawn human spinal fluid. When Randall uses to help him by providing the fluid, Brewster and his captive gorilla must attempt to find an appropriate donor.